added support for new HiDPI retina Macs (closes #303)

Via the new property "supportHighResolutions" on the Starling instance.
added check if 'stage.contentsScaleFactor' is available
Fixed bug where vertexID was being used instead of zero

getBounds() function would not work when specifying numVertices that is smaller or equal to vertexID. This was because vertexId was being used incorrectly in the for loops. Zero should have been used instead
Merge pull request #309 from hamidhomatash/patch-1

Fixed bug where vertexID was being used instead of zero